1. Field of the Invention
Embodiments of the present invention relate to an image forming device that can control a position of an intermediate transfer body in a width direction. Specifically, the embodiments relate to the image forming device that can reduce downtime by controlling the position of the intermediate transfer body in the width direction.
2. Description of the Related Art
In an image forming device having an intermediate transfer belt, a toner image is transferred from a photosensitive drum onto the intermediate transfer belt by rotating the photosensitive drum while circulating the endless intermediate transfer belt. In a color image forming device, the above process is repeated four times, and a color toner image is formed on the intermediate transfer belt by superimposing four toner images on the intermediate transfer belt. Therefore, it is important that positions of the four toner images in the circulating direction and in the width direction of the intermediate transfer belt accurately coincide with each other.
A technique for causing the positions of the four toner images to coincide has conventionally been known (c.f. Patent Document 1 (Japanese Published Unexamined Application No. H11-102098)). Patent Document 1 discloses an image forming device that optically detects plural marks for detecting positional shifts formed on the intermediate transfer belt, and that corrects the positional shifts based on the spacing among the plural marks for detecting the positional shifts.
Further, the intermediate transfer belt rotates while suspended by plural rollers. Thus, when a primary transfer roller or a secondary transfer roller contacts the intermediate transfer belt, a force acts on the intermediate transfer belt along an axis direction of a transfer roller by the impact of the contact. When the force acts on the intermediate transfer belt, the intermediate transfer belt may start moving in the axis direction of the transfer roller so as to find a stable rotational position (hereinafter, this movement is referred to as a width direction movement), due to an error on placing the transfer roller or tension of the intermediate transfer belt. When the width direction movement occurs, the whole intermediate transfer belt slowly moves in the main scanning direction.
It is preferable that a position of the intermediate transfer belt in the width direction be set to be at a target position. As a method of correcting the position of the intermediate transfer belt in the width direction, the steering method has conventionally been known. In the steering method, the position of the intermediate belt in the width direction is controlled by varying tilts of the rollers that suspend the intermediate transfer belt (hereinafter, referred to as the steering rollers).
An image forming device that can control the position by the steering method has an edge sensor that detects an end portion of the belt in the width direction (hereinafter, referred to as the edge position). The image forming device compares the edge position and the target position, and controls the tilts of the steering rollers depending on the result of the comparison.
When the steering method is adopted, since the intermediate transfer belt is caused to be moved to a stable target position, a moving amount in the width direction with respect to the target position eventually becomes zero. However, it has been known that, when an image is formed while the intermediate transfer belt is moving in the width direction, the quality of the image is significantly degraded. Therefore, when the primary transfer roller or the secondary transfer roller contacts the intermediate transfer belt, the image forming operation is temporarily stopped. The image forming operation is restarted, after waiting for the position of the intermediate transfer belt in the width direction to be stabilized.
Here, it takes a few seconds to a few minutes for the image forming device to correct the moving amount in the width direction by using the steering method. The time interval for controlling the position of the intermediate transfer belt in the width direction is downtime for the image forming device.
When the downtime is lengthened, the printing efficiency of the image forming device is lowered. To overcome this problem, a method has been developed to reduce the downtime by changing the target position of the intermediate transfer belt in the width direction, so as to quickly stabilize the position of the intermediate transfer belt in the width direction (c.f. Patent Document 2 (Japanese Registered Patent No. 4244594)).